People do fall in love
At work and, once that happens, their working relationship becomes tinted with personal aspects.
Anyone claiming it doesn't happen is either in denial or an idiot.
However, the remedy to this is not in a single-sex environment but in dealing with the particular couple as and when that happens. What specifically should be done depends on the individuals and the circumstances. Transfer one to a different lab, dismiss both, or let things remain unchanged? - these are the usual managerial choices and they haven't changed for good 100s of years. They work and they don't need reinventing the wheel.
I'm saying all this as someone who has a [very happy] personal experience with all 3 "things" as well as with having to "solve" the problem - in an office, rather than in a lab, admittedly, though.